Lori Daniels


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Postdoctoral Fellows

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2021)
Drought influences mixed-severity fire regimes across temporal and spatial scales in the Montane Cordillera of Canada (2019)

Understanding historical fire-drought associations, particularly in forests with mixed-severity fire regimes, is a research and fire management priority in western North America. My thesis investigates how drought variation across temporal and spatial scales drove such fire regimes in the Montane Cordillera of Canada. I developed three-interrelated studies written as independent chapters, all of which used crossdated fire-scars to represent historical fire years. The first two studies test fire-drought associations using monthly adaptations of the Drought Code (DC) from Canada’s Fire Weather Index System. First, I compared three monthly drought codes during the 20th and 21st centuries for montane forests of southeast British Columbia. Accuracy of monthly DC increased after accounting for overwinter drying, early fire season starts, and effective precipitation. June-August drought codes were significantly associated with historical fires. Variation in fire-season drought influenced fire severity, connecting modern fire-weather indices with historical mixed-severity fire regimes. Second, I investigated how historical drought variation drove mixed-severity fire regimes in the same location by developing a tree-ring proxy reconstruction of summer DC. Comparing summer DC against a local summer Palmer Drought Severity Index provided a nuanced understanding of inter-annual fire-drought associations and moisture content among forest fuels, namely in deep compact organics in the soil and large woody fuels, versus the duff layer. Fire years were associated with coinciding and previous year summer drought; but limited by coinciding and previous year summer wet conditions. Summer moisture conditions during fire years likely influenced ignitions and led to variable combustion of forest fuels. The final study encompassed broader spatial coverage by including 17 fire-history sites across the Montane Cordillera, and by testing historical associations between climate and fire based on years with evidence of fire at multiple sites, i.e., fire synchrony. Fire synchrony was historically common, and associated with droughts at regional and subregional scales based on tree-ring proxy reconstructions of climate. My thesis provides information on drought as a driver of mixed-severity fire regimes across temporal and spatial scales. Ultimately, understanding how drought drove mixed-severity fire regimes across scales, helps fire managers anticipate how these fire regimes are shifting due to climate change.

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Understanding Disturbance, Facilitation, and Competition for Conservation of Whitebark Pine in the Canadian Rockies (2012)

Understanding forest resilience to novel disturbances and how tree interactions will be affected by global change is critical for predicting future forest composition. The widespread decline of the endangered whitebark pine (Pinus albicaulis) in the Canadian Rockies due to non-native white pine blister rust (Cronartium ribicola) and native mountain pine beetle (Dendroctonus ponderosae) and Ips sp. permitted examination of interactions between disturbances and tree responses in high-elevation forests. Disturbance severity was high with 20-90% whitebark pine mortality over 50 years in 16 stands. Basal annual increment (BAI) of whitebark pines prior to mortality from mountain pine beetles declined 46%, but only by 25% for those subsequently killed by blister rust and Ips sp.. Climate-growth relationships suggest blister rust increased sensitivity of whitebark pines to variation in summer precipitation, reducing resistance to beetles. The mortality of whitebark pine was used as an in-situ experiment simulating neighbour removal to test the stress-gradient hypothesis of tree interactions. Facilitation intensity, determined by comparing subalpine fir (Abies lasiocarpa) regeneration around live, top-killed, or dead adult whitebark pines, increased with elevation but depended on benefactor size and neighbourhood density. Large-diameter, top-killed whitebark pines were more facilitative than live trees, indicating thresholds in benefactor size, below which live, healthy trees were facilitators and above which they were competitors. Size thresholds were also found in interactions between adult trees where competition intensity increased between trees of greater diameter differences as indicated in BAI releases of subalpine fir after the death of neighbouring trees. Conversely, the importance of competition relative to other factors influencing growth increased between trees more similar in size and with abiotic stress. My results refine the stress-gradient hypothesis by demonstrating hierarchical influences on tree interactions. The predominant release from competition doubled subalpine fir’s BAI from the landscape average pre- disturbances compensating for the decline predicted by climate-growth relationships. Lack of regeneration and growth release in surviving whitebark pines and an abrupt shift in key variables suggest a regime shift to fir dominance and whitebark pine extirpation. Whitebark pine resilience was higher at sites of low abiotic stress and disturbance severity, relationships useful for conservation.

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Growth release of trees following fine-scale canopy disturbances in old-growth forests of coastal British Columbia, Canada (2008)

Growth release of trees following canopy disturbances is of interest to ecological scientists and forest managers. Using dendroecological techniques, I examined growth release of canopy and subcanopy trees following the formation of natural, fine-scale canopy gaps in old-growth, western red cedar-western hemlock forests of coastal British Columbia. I aimed to quantify detailed information on release of the three shade-tolerant tree species that constitute these stands: western red cedar (Thuja plicata), western hemlock (Tsuga heterophylla), and Pacific silver fir (Abies amabilis).As a first step, I calibrated the radial-growth averaging method to account for regional-scale variability and capture a more complete range of growth releases that may occur following the formation of fine-scale gaps in the study stands. A 25% threshold, 5-year moving average, and 10-year window emerged as appropriate parameters for detecting releases using radial-growth averaging. Basal area increment was also the most appropriate growth index for detecting releases. Establishing these empirically-based criteria was important for quantifying the magnitude and duration of releases.Tree diameter and growth rate prior to release were the most important predictors of the magnitude and duration of releases, but identity of the tree species and distance from the gap center were also important predictors. Western hemlock and Pacific silver fir were often growing slowly both in the canopy and subcanopy, giving them tremendous potential to release. For these species, releases were generally intensive and persistent. In contrast, western red cedar were often growing quickly both in the canopy and subcanopy, giving them less potential to release. Compared to western hemlock and Pacific silver fir, western red cedar releases were less intensive and persistent. Patterns related to distance from the gap center emerged for trees growing along the north-south axis of gaps. Regardless of species, increasing distance from the gap center resulted in decreasing magnitude and duration of releases. However, patterns for duration were complex, as the distance effect was greater for trees north of the gap center.Information on growth release of trees is useful for reconstructing the history of past canopy disturbances, elucidating mechanisms of tree species coexistence, and assessing and predicting stand changes due to forest management in coastal British Columbia.

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Master's Student Supervision (2010 - 2020)
Freezing to death in a warming climate: drivers of yellow-cedar decline on Haida Gwaii (2019)

The global rise in temperature and associated changes in climate have led to decline of forests around the globe, across multiple species and ecosystems. This includes yellow-cedar (Callitropsis nootkatensis) decline, which is one of the most severe in North America. I found abundant evidence of tree decline and mortality of yellow-cedar on Haida Gwaii across multiple watersheds and over a range of elevations. The decline on Haida Gwaii parallels the broader yellow-cedar decline in terms of spatial distribution, symptoms, magnitude and timing. Nevertheless, the proposed drivers on the mainland may not adequately explain the decline on Haida Gwaii, due to the more temperate climate and lack of persistent snowpack. I investigated several possible drivers both at the local and regional scale. My results are inconsistent with stand dynamics as a driver of elevated decline and mortality. Neither increased competition, nor aging of a cohort explains the decline. Onset of basal area increment decline and mortality have been accumulating over time, with increased rates since the 1980s. Alternatively, the magnitude and timing of the decline is consistent with well-documented multi-decadal variations and long-term directional trends in regional climate. I found patterns of divergent growth and divergent response to climate among yellow-cedars at the same sites. Yellow-cedars affected by decline were decreasing in growth and negatively associated with warmer drier winter conditions. Whereas, yellow-cedars not affected by decline were increasing in growth and positively associated with warmer growing season temperatures. The limiting factors for declining trees, warm dry winter conditions, are consistent with the hypothesis from the mainland that climate warming has led to root freezing. However, compared to drivers of yellow-cedar decline on the mainland, snowpack plays a less important role on Haida Gwaii. Warming temperatures likely have a more direct effect. I propose warmer winter temperatures have led to a combination of decreased cold-hardening and earlier dehardening, in conjunction with increased frequency of thaw-freeze cycles and freeze events during otherwise milder winters. This exposes yellow-cedar’s fine roots to varying degrees of freezing damage over multiple winter thaw-freeze cycles, causing physiological stress, tree decline and eventual death.

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Historical and contemporary disturbance regimes in central interior dry forests of British Columbia (2019)

Accurate historical reconstructions of disturbance regimes are necessary because current approaches toward more sustainable forest practices attempt to model forest management after historical disturbance regimes and recovery processes. Using dendrochronological analyses, I characterized both the historical fire regime and outbreaks of western spruce budworm (Choristoneura occidentalis Freeman) within a dry mixed-conifer forest to assess whether contemporary regimes are outside the historical range of variation. By determining the degree of change between the historical fire regime and the current, I provide insight into the potential consequences that fire suppression has had on resilience to fire and budworm. Historical fires burned in 23 different years from 1619 to 1943 with a mean interval of 15 years. Beginning in the later part of the 19th century fire patterns changed, such that the current fire-free period drastically exceeds the historical maximum fire-free intervals at all but 1 plot. Fire scars and post-fire cohorts varied among plots and indicated fires burned at mixed severities through time and space. Eight western spruce budworm outbreaks initiated between 1800 and 2001 with mean a mean duration of 14 years and mean return interval of 29 years. Outbreaks of a range of durations, frequencies, and severities were well represented throughout the record with no discernable trend in time. There was no significant difference between the number and timing of infestations initiating in plot canopies or subcanopies. The simple linear regression analyses determined plot-level subcanopy tree density (R² = 0.02; p = 0.43,) and the ratio of subcanopy to canopy tree densities (R² = 0.02; p = 0.48) to be poor predictors of severity during the outbreak from 2001 to 2011. It is strongly suspected that in the absence of fire in the 20th century, the forest has become denser and has lost a substantial degree of structural diversity. In addition, accumulations of ladder and surfaces fuels have also occurred and leave the forest at increased risk of a high-severity stand-replacing fire and less resilient to budworm attack.

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Humans, climate, and an ignitions-limited fire regime at Vaseux Lake (2017)

This study investigated the role of human land use and climate as drivers of the historical fire regime of a 400ha protected area in the Okanagan region of British Columbia. I used fire scars and forest demography data to reconstruct spatiotemporal patterns in fires from 1714 - 2013. I also used paleo-climate reconstructions derived from tree ring series to evaluate whether historical fire-climate relationships changed with the displacement of indigenous peoples. Fire patterns were closely coupled with the human history of the study area. Fires were more frequent, less synchronous, and burned earlier in the season when indigenous people were stewarding the study area traditionally. Logistic regression showed that fires were also twice as likely during this period, and that topographic factors were not a significant control of the fire regime. Analysis of fire-climate relationships revealed that human land use superseded the effects of inter-annual and decadal-scale climate as a driver of historical fires. Fires occurred during a variety of conditions when indigenous people were stewarding the study area traditionally, while fires after indigenous people were displaced were associated with El Niño years, which tend to bring warm/dry conditions to the region. The historical fire regime at Vaseux was of mixed-severity in time and space, and this variability helped generate a complex forest structure. Historical fires acted to control tree establishment and mortality, and the forest is now denser than it was historically due to reduced fire frequency in the late 20th century. Continued infilling could shift the fire regime towards a greater component of high-severity fire. The results suggest that indigenous traditional land stewardship was the dominant control of historical fire dynamics at Vaseux. Managers wishing to preserve habitat and forest structures generated by the historical fire regime will need to account for the influence of indigenous burning, and modern lightning intervals will not be a sufficient baseline for setting treatment intervals. Proactive management designed to maintain a fire regime of frequent mixed-severity fires will be necessary to promote ecological resilience in an uncertain future.

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The effects of single-objective management on disturbances in central interior dry forests of British Columbia (2017)

Mule deer are an important game species, and have become the focus of applying a particular silvicultural treatment that enhances habitat while allowing timber harvesting. Mule deer winter range management (MDWRM) involves the proportional removal of trees based on their diameter and abundance resulting in a multilayered, Douglas-fir dominated forest with a clumpy tree distribution. I assessed changes in forest stand attributes brought about by MDWRM through time and how these attributes related to stand susceptibility to the western spruce budworm, Douglas-fir beetle, and wildfire using a randomized complete block single factor mixed-effects model with subsampling. In the short-term, MDWRM significantly changed (p
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Fire history and climate-fire relations in Jasper National Park, Alberta, Canada (2014)

In mixed-conifer forests of western North America, fire ecologists and managers are increasingly recognizing the prevalence and importance of mixed-severity fire regimes. However, these fire regimes remain poorly understood compared to those of high- and low-severity. To enhance understanding of fire regimes in the montane forest of Jasper National Park (JNP), I reconstructed fire history and assessed forest composition, age and size structure at 29 sites (Chapter 2). Historic fires were of mixed severity through time at 18 sites, whereas the remaining 11 sites had evidence of high-severity fires only. At the site level, mean importance values of canopy trees were more even among coniferous species and greater for Pseudotsuga menziesii at mixed-severity sites. The greater numbers of veteran trees and discontinuous age structures were also significant indicators of mixed-severity fire histories.In a second study, I crossdated tree ages and fire-scar dates for 172 sites and tested whether historic fire occurrence depended on inter-annual to multi-decadal variation in climate (Chapter 3). Eighteen fires between 1646 and 1915 burned during drought years, with a weak association to El Niño phases and the negative phase of the Pacific Decadal Oscillation. Fire frequency varied through time, consistent with climate drivers and changes in land use at continental to inter-hemispheric scales. No fire scars formed since 1915, although potential recorder trees were present at all sites and climate was conducive to fire over multiple years to decades. Thus, the absence of fires during the last century can largely be attributed to active fire suppression. Improved understanding of the drivers of the historic mixed-severity fire regime enhances scientifically-based restoration, conservation, forest and wildfire management in the Park and surrounding montane forests.

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Historical fire regime of the Darkwoods : quantifying the past to plan for the future (2011)

This study quantifies the fire history of the Darkwoods; a 55,000 ha property in the South Selkirk Natural Area of southeastern British Columbia, owned and managed by the Nature Conservancy of Canada. Fire scar and tree cohort chronologies from 45 plots, extending from the years 1406 – 2010, were used to determine the temporal and spatial variability of historic fires in ~4,000 ha of the southeastern-most watershed of the property, and to assess the accuracy of provincial Natural Disturbance Type (NDT) classes for the study area. In light of a mixed-severity fire regime, new and novel methods of historic fire mapping using Inverse Distance Weighting methods in a GIS were also analyzed.Using logistic regression, the spatial variation of fires at the tree- and plot-levels differed greatest by elevation, but fires at the tree-level also varied by slope steepness and slope aspect. Anthropogenic influences on the occurrence of fire over time were also evident, but only after 1945, when the occurrence of fire dropped significantly likely due to the introduction of modern methods of fire suppression in the 1940s. Results indicate a mixed-severity fire regime for the study area, and the presence of numerous fire scars in mid- and high- elevation plots, in conjunction with mean fire return intervals less than 100 years, provide evidence that conflicts with provincial NDT designations.Including high-elevation stand ages, determined from increment cores, provided evidence of the absence of fire and helped refine estimates of fire boundaries, particularly in and around areas experiencing mixed- and high-severity fires. Spatial Mean Fire Intervals were longer than those calculated at the tree-, plot- and watershed-levels, reflecting the degree to which a mix of high-severity, stand-replacing fires, with low- and moderate-severity, stand-maintaining fires, can lengthen mean return intervals across a mixed-fire landscape.

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Decay Dynamics of Coarsewood Habitat in Old-growth Spruce and Pine Stands in the Rocky Mountain Foothills (2010)

This thesis presents research on the decay dynamics of coarsewood wildlife habitat in the foothills of the Rocky Mountains, west-central Alberta. The study sites were located in permanent sample plots in five Picea glauca and five Pinus contorta old-growth stands. I combined field sampling, dendrochronology, and permanent sample plot data to characterize snags and logs. I used a functional classification scheme to assess the potential wildlife habitat value of snags and logs. The study had two main objectives: (1) to quantify the magnitude of error in dendrochronological work on decayed wood and (2) to assess the accumulation and persistence of snags and logs and their potential functions as wildlife habitat.I used permanent plot data to verify the accuracy of year-of-death estimates obtained by crossdating snags and logs. I obtained YOD estimates from 71 snags and 54 logs. Most YOD dates occurred within the observed interval of death dates from the permanent plot data (54%-80%, grouped by species and coarsewood type) and most remaining dates preceded the interval of death. Overall, the magnitude of error in YOD estimates increased with time since death.I located 322 snags and 405 logs. Mean densities were 403 snags/ha and 506 logs/ha. Snags and logs in intermediate decay classes were the most common, and I hypothesize that most snags reach decay class 4 or 5, rot at the base and fall over, rather than decaying completely in situ. Coarsewood persisted for many decades after death: estimated time since death of the oldest snag and log was 180 and 175 years, respectively. Time since death varied significantly across decay class, but the range of YOD dates in each decay class was so broad that decay class was not a reliable indicator of approximate time since death. Most observations of habitat functions were limited to one of five functional types. Less than 1% of snags and 4% of logs provided four or more habitat functions.Given the longevity of coarsewood in these stands, management plans must take a long-term view in order to maintain levels of coarsewood that are within the natural range of variability.

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Quantifying forest fire variability using tree rings Nelson, British Columbia 1700-present (2010)

This study uses dendroecology to provide direct evidence of historic forest fires and their effects on stand structure and dynamics at a local scale in the montane forests in southeastern British Columbia (BC). Using tree ages and fire-scarred trees, I determined the historic variability of fires by quantifying stand dynamics in relation to past fires in the mixed-conifer forests surrounding Nelson, a wildland-urban interface community in southeastern BC. I built fire records that extended from 1642–2009 across 18 sites in the ~160,000 hectare study area. Although a watershed-level fire signal is evident, site-to-site differences in fire-scar records and stand dynamics suggest that topography and land use caused variability in the fire histories of the individual sites. Numbers of fire-scarred trees and importance values of fire-tolerant trees decreased significantly with elevation. Fire-intolerant trees were most abundant in the subcanopy across all elevations. Most strikingly, no fires were recorded since 1932 across all sites, suggesting that fire exclusion has been effective and that future stands will likely continue to diverge from historic stands by becoming more dense, more homogenous in species composition, and, as a result, more susceptible to high-severity fires.

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